As used herein, a heterostructure is a series of layers, some of which may be epitaxial, that may be chosen to allow electron or hole confinement in one or more of the layers. Epitaxial means that that the crystal structure is not interrupted at the interface between layers. A quantum heterostructure is a heterostructure in a substrate (i.e., semiconductor) where size restricts the movements of charge carriers forcing them into a quantum confinement. This leads to the formation of a set of discrete energy levels at which the carriers can exist.
A quantum confined semiconductor can be defined based on the number of electron or hole confinement dimensions. A quantum dot defines electron or hole confinement in all three dimensions. A quantum wire defines electron or hole confinement in two spatial dimensions while allowing free propagation in the third dimension. A quantum well defines electron or hole confinement in one dimension while allowing free propagation in two dimensions. By doping a quantum well or the barrier of a quantum well with donor impurities, a two-dimensional electron gas (2DEG) may be formed. Alternatively, acceptor dopants can lead to a two-dimensional hole gas (2DHG).
In general, quantum wires, wells, and dots are grown using epitaxial techniques in nanocrystals produced by chemical methods or by ion implantation or in nanodevices produced using lithographic techniques. The energy spectrum of a quantum dot can be engineered by controlling the size, shape, and strength of the confinement potential.
Quantum dot technology is one of the most promising candidates for use in solid-state quantum computing. Quantum computing utilizes quantum particles to carry out computational processes. The fundamental unit of quantum information is called a quantum bit or qubit. By applying a voltage to one or more gates formed on the quantum heterostructure, the flow of electrons through the quantum dot can be controlled and precise measurements of the spin and other properties of the electrons can be made. A qubit is a two-state quantum-mechanical system that includes an “on” state, an “off” state, and interim states that are superpositions of both the on and off states at the same time. In a quantum dot, the on/off state can be associated with an up/down spin or an energy state of the electron(s) in the quantum dot.
Quantum heterostructure previously included a multiple layer semiconductor structure having a substrate, a back gate electrode layer, a quantum well layer, a tunnel barrier layer between the quantum well layer and the back gate, and a barrier layer above the quantum well layer. Multiple electrode gates are formed on the quantum heterostructure with the gates spaced from each other by a region beneath which quantum dots can be defined. Appropriate voltages applied to the electrode gates allow the development and appropriate positioning of the quantum dots.
For illustration, a double quantum dot 100 is shown with reference to FIG. 1. Double quantum dot 100 includes a source region 102, a drain region 104, a left dot gate 106, a right dot gate 108, a left quantum dot region 110, and a right quantum dot region 112. Quantum control of electrons in double quantum dot 100 is provided primarily by adjusting the energy of electrons in left quantum dot region 110 and right quantum dot region 112 and the tunnel rate of electrons into and out of left quantum dot region 110 and right quantum dot region 112. The adjustments can be made through application of a voltage to one or more of left dot gate 106 and right dot gate 108, which change the energy within left quantum dot region 110 and right quantum dot region 112.
For further illustration, FIG. 2 shows a scanning electron microscope (SEM) image of a double quantum dot device 200 described in C. B. Simmons, et al., Tunable spin loading and T1 of a silicon spin qubit measured by single-shot readout, Phys. Rev. Lett. 106, 156804 (2011). Double quantum dot device 200 is a charge qubit fabricated in a Si/SiGe heterostructure. Changes in the charge states of left quantum dot region 110 and right quantum dot region 112 can be observed through measurement of a current through drain region 104 in response to voltage pulses applied to a first control gate 206. The location of tunnel barriers between source region 102 and left quantum dot region 110, between left quantum dot region 110 and right quantum dot region 112, and between right quantum dot region 112 and drain region 104 are indicated by the labels A, B, and C, respectively in FIGS. 1 and 2. Labels A, B, and C represent barriers an electron must tunnel through to travel from source region 102 to drain region 104.
In devices such as double quantum dot device 200, the gates are spatially far apart from one another, and quantum dot confinement is controlled by pinching off the intervening open channels of electrons from the sides. This pinch-off behavior adjusts the tunnel barrier height around a quantum dot as illustrated in FIG. 3. FIG. 3 shows a tunnel barrier height as a function of lateral position (from A to C) for different voltages applied to first control gate 202. A first tunnel height curve 300 results from a first voltage applied to first control gate 202. A second tunnel height curve 302 results from a second voltage applied to first control gate 202. A third tunnel height curve 304 results from a third voltage applied to first control gate 202. The third voltage is more negative than the second voltage which is more negative than the first voltage. Thus, FIG. 3 illustrates pinching off of the conduction channel at tunnel barrier A using a negative voltage applied to first control gate 202.
In traditional semiconductor dot device designs, the gates are placed directly on the surface of the heterostructure and leakage from the gate to the 2DEG is prevented by a Schottky barrier that forms between the gate and the heterostructure. Schottky barriers are only insulating under negative bias, so the electrostatic gates described with reference to FIGS. 2 and 3 can only have negative voltages applied. Furthermore, the mere presence of the gates on the surface depletes the electrons in the 2DEG underneath. This effect prevents gates from being placed too close to the desired tunnel barriers A, B, C between left quantum dot region 110 and right quantum dot region 112, since by doing so the fast tunnel rates required for device operation might not be achievable.
While these open designs have led to many successful devices for manipulating one to four quantum dots, it is challenging to tune tunnel rates while leaving the quantum dot energy levels fixed. The reason for this difficulty is that two nearby gates have similar couplings to proximal tunnel barriers and quantum dots. Thus, changing the tunnel rate while leaving the dot energy fixed is not achievable by changing the voltage applied to a single gate, but rather involves changing the voltages of multiple nearby gates in a complicated compensation process that becomes more challenging as the number of dots in a device increases.